Data from resonant photoluminescence excitation spectroscopy experiments performed on the monolayer transition metal dichalcogenide (TMDC) MoSe2 are presented. The excitation laser is scanned across the exciton and trion optical frequencies, and at each excitation wavelength a spectrum of the emitted light is measured. This data gives unique insight into the homogenous and inhomogenous broadening present in monolayer TMDC materials. In monolayer MoSe2 samples exfoliated on to glass or SiO2 on silicon, we measure homogenous linewidths of ~2 meV. The homogenous linewidth is much more uniform than the inhomogenous linewidth, both between and within flakes. Encapsulation in hexagonal boron nitride somewhat reduces the homogenous linewidth, possibly by some combination of altering the coupling of the exciton to phonon modes and altering the radiative coupling to the environment. We also investigate the degree of linear polarization of the emitted light as a function of both excitation and emission wavelength. This gives insight into the decoherence between the superposition of excitons created at both the K and K'points under excitation with linearly polarized light.

Tellurium is a hexagonal chiral crystal with covalently bonded left- or right-spiral chains of atoms along the c-axis, with much weaker van der Waals interactions between the chains. Here we use transient polarized reflectivity measurements to study this complex band structure and carrier dynamics at 10 K and 300 K from 0.3 to 1.2 eV (4000 to 1000 nm). The transient spectra reveal a series of transitions with strong polarization anisotropy. We see optical transitions between all three VB (H4, H5 and H6 symmetries) with the doubly degenerate CB (H6 Symmetry). While the ground state energies are stable with time, higher lying states exhibit dynamic shifts to lower energy as a function of pump delay, showing strong correlation with relaxation processes of photoexcited carriers. The main decay of photoexcited carriers occurs within the first 60 ps, followed by a weak long-lived decay which perhaps suggest some inter-valley scattering before recombination. These results will help in understanding photoconductive properties in Te-based novel devices for thermoelectric as well as optoelectronic applications.

Dark excitons, XD, in semiconductors are appealing candidates for quantum computing and optoelectronics owing to their long lifetimes. However, optical read-out and control of XD states has remained challenging due to their decoupling from light. Here, we present a tip-enhanced nano-optical approach to induce and control the radiative emission of XD at room temperature. Using a monolayer (ML) WSe2 on a gold substrate, we demonstrate ~6×105-fold photoluminescence (PL) enhancement through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with large Purcell factor of ≥2×103 of the tip-sample nano-cavity. As an extension experiment of the tip-enhanced PL (TEPL) of WSe2 ML, we control the radiative emission of localized exciton (XL) with 10 nm spatial resolution nano-imaging to understand unique quantum emitting properties of XL in space and time. Our approach provides a new and facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.

Quasiparticle interference (QPI) measured by low-temperature scanning tunneling spectroscopy (STS) accompanied by a Fourier transform analysis (FT-STS) provides unique insight into the effect of atomic defects on carrier scattering inside and between valleys in two-dimensional (2D) materials. Here I present a general T-matrix based framework for the calculation of FT-STS spectra which in combination with atomistic DFT calculations of the defect scattering potential allows for detailed FT-STS modeling of "realistic" defects such as, e.g., atomic vacancies, adatoms and substitutional atoms [1].

In monolayer transition metal dichalcogenides (TDMs; MX2), atomic vacancies are commonly believed to be a source of pronounced intervalley scattering, thereby presenting a serious obstacle for applications exploiting their unique valley-contrasting properties. However, as I here show, the symmetry of the defect site gives rise to selection rules which may protect against intervalley scattering. In the conduction-band FT-STS spectra this manifests itself by a K ↔ K' intervalley peak which is missing for X vacancies, while appearing clearly for M vacancies. These findings put the recent observations of absent K ↔ K' intervalley peaks in QPI experiments [2,3] in a new perspective.

In graphene, the chiral nature of the states leaves clear fingerprints in the FT-STS spectra [4,5]. For example, the q=2kFring due to backscattering is strongly suprressed near the Dirac point where trigonal warping is small -- this in spite of the fact that atomic defects often break the A,B sublattice symmetry thus allowing for backscattering. The explanation for this apparent paradox emerges straight forwardly from our unified theory.

Symmetry protected topological (SPT) states in two-dimensions (2D) exhibit counter-propagating edge modes with well-defined quantum numbers. The symmetry is important for the robust longitudinal quantization as it prevents back-scattering. These states appear in Dirac semi-metals in the quantum Hall regime at the charge neutrality point and in 2D topological insulators. In this talk, I will discuss the role of interactions and disorder on the edge excitations and longitudinal conductance of SPT states in 2D. The longitudinal conductance is robust to intra-edge and inter-edge interactions and symmetry preserving edge disorder. However, symmetry breaking edge disorder results to localization of the edge modes. The localization length depends on the topological properties of the SPT states. I will also discuss the formation of exotic ordered states along the edges induced by interactions and symmetry breaking edge disorder.

Two-dimensional transition-metal dichalcogenide (TMD) monolayers exhibit direct bandgap excitons with large binding energy. The optical response of TMDs is electrically tunable over a broad wavelength range, making these 2D materials promising candidates for optoelectronic devices. In this work, we integrate TMDs with various nanostructures fabricated on single crystalline silver thin films, and study the coupling between excitons in TMDs with surface plasmon polaritons (SPPs). We show that the coupling of exciton emission into SPPs can be utilized as to directly probe the dipole orientation of excitons in various TMDs. We further enhance the light-matter interactions by fabricating plasmonic crystal cavities, and demonstrate electrostatically tunable vacuum Rabi splitting in such a system.

Molybdenum disulphide (MoS2) plays a central role in the burgeoning field of two-dimensional (2D) materials. Particularly, it has extraordinary optical properties which relies on strong electron-hole interactions. However, experimental reports as well as fundamental understanding of the optical properties isolating the roles of both defects and extrinsic environmental factors are still lacking for MoS2. We have performed in situ spectroscopic ellipsometry in a wide spectral range and under ultra-high vacuum (UHV) conditions to explore the complex dielectric function of MoS2. Combining high temperature UHV annealing and oxygen exposure, we show tunability of the exciton and trion features of the optical spectra. We attribute these tunability to desorption and chemisorption of oxygen upon UHV annealing and oxygen exposure, respectively. Ab initio density functional theory calculations suggest that oxygen chemisorption passivates gap states originating from sulphur vacancies, which influences the recovery of the exciton and trion features. Overall, this study highlights the importance of adsorbed oxygen as well as defects in the interpretation of experimental results from MoS2.

We investigate low temperature photoluminescence of interlayer excitons in hexagonal boron nitride (hBN) encapsulated monolayer MoSe2 and WSe2 vertical heterostructure. Surprisingly, we find the interlayer exciton splits into four different resonances at 1305 meV, 1340 meV, 1360 meV, and 1387 meV. All four resonances exhibit long photoluminescence lifetimes of hundred nanoseconds, which verify their interlayer nature. Based on the temperature and excitation power dependence, we tentatively assign the lower energy resonances, 1305 meV and 1340 meV resonances to be indirect and direct interlayer exciton in the momentum space, respectively.

Two dimensional transition metal dichalcogenide (TMD) monolayers are direct bandgap semiconductors that feature tightly bound excitons. TMDs can be easily integrated with various functional substrates or materials through van der Waals stacking. For these reasons, TMDs are particularly interesting both for novel device applications as well as studies of fundamental physics. In this work, we show that a monolayer of the TMD MoSe2 encapsulated by hexagonal boron nitride can reflect up to 85% of incident light at the excitonic resonance. We study the root of this high reflectance: the excellent coherence properties of excitons in this atomically thin semiconductor. Moreover, we show that power and wavelength dependent nonlinearities in these systems stem from exciton-based lattice heating and exciton-exciton interactions.

The 2D geometry nature and low dielectric constant in transition metal dichalcogenides (TMDCs) lead to easily formed strongly bound excitons and trions. We studied the photoluminescence of van der Waals heterostructures of monolayer MoS2 and graphene at room temperature and observed two photoluminescence peaks that are associated with trion emission. Further study of different heterostructure configurations confirms that these two peaks are intrinsic to MoS2 and originate from a bound state and Fermi level, respectively, of which both accept recoiled electrons from trion recombination. We demonstrate that the recoil effect allows us to electrically control the photon energy of trion emission by adjusting the gate voltage. In addition, significant thermal smearing at room temperature results in capture of recoil electrons by bound states, creating photoemission peak at low doping level whose photon energy is less sensitive to gate voltage tuning. This discovery reveals an unexpected role of bound states for photoemission, where binding of recoil electrons becomes important.

Inspired by the discovery of graphene, an array of 2D materials have been isolated that exhibit a variety of interesting electronic properties. One such class being the semiconducting transition-metal dichalcogenides (TMDCs) that support optical transitions ranging from visible to NIR. Quantum confinement in these monolayers leads to strong exciton and trion resonances even at room temperature. Furthermore, the TMDCs exhibits symmetry mediated optical selection rules that couples the two inequivalent Brillouin zone K points with distinct circular polarization states. This property offers the realization of valleytronic devices. Availing the recently discovered localized quantum dot-like emissions in TMDCs, we have demonstrated controlled charging in these individual confined excitons in monolayer WSe2 in a charge tunable van der Waals heterostructure. We show that these quantum dot trions can also inherit the polarization selection rules of the host material under certain conditions. The trion ground state is important for localized single spin studies in TMDCs. This shows potential for using TMDCs as building blocks for quantum photonic circuits that possess optically accessible ground-state spins.

Atomically thin monolayers of transition metal dichalcogenides supports strongly bound excitons in two dimensions. In this work we use monolayer of MoSe2 as an active material inside a planer microcavity structure to show strong coupling between exciton and cavity photon. Unintentional doping in the exfoliated monolayer results into formation of trions, the bound state of an electron and exciton. We observed an inverted trion dispersion with negative curvature near zero in-plane momentum. Our theoretical analysis show that such negative curvature in the dispersion or the negative mass can only arise due to electron mediated interaction between the lower exciton-polariton branch and the trion-polariton. Our work suggest a pathway for studying interesting regimes in quantum many-body physics yielding possible new phases of quantum matter.

Van der Waals (vdW) heterostructures built of 2-dimensional (2D) materials, such as single layer transition metal dichalcogenides (TMDs) and boron nitride (h-BN), have generated wide interest to investigate novel optoelectronic devices. The large excitonic binding energy of TMDs and their intrinsic 2D nature allow for interesting ways to explore novel quantum optical effects in TMDs. We will discuss our recent results of vdWs heterostructures formed by stacking together two different TMDs (a type-II heterostructure) encapsulated with h-BN with electrical contacts and dual gate configuration. Using an optical excitation, we generate excitons with the electron and the hole each residing in the two different TMDs (interlayer excitons, IE). Thus, IEs have a dipole moment oriented out-of-plane and are repulsive in nature, because of the Coulomb interaction. With increasing excitation power, we create a large density of IEs (5x1011 cm-2) and observe long diffusion ~ 20µm even at elevated temperatures (T = 60K). Because the IEs diffuse from areas of larger density and temperature (the excitation spot), to regions outside the hot generation spot, they create a cold gas of bosons. A large density of IEs is important for novel optoelectronic devices such as IE condensates and lasers.